Stern drive unit trim tab

ABSTRACT

Disclosed herein is a stern drive lower unit comprising a lower portion which is at least partially submerged during normal operation and which includes a rotatably mounted propeller shaft carrying a propeller, and a trim tab which is located rearwardly and above the propeller and which includes two side surfaces, one of which is subject to thrust or impact by water propelled by the propeller and is provided with a forward portion and a rearward portion offset from the forward portion in the direction toward the other of the side surfaces.

United States Patent [1 1 Granholm [4 1 Mar. 26, 1974 STERN DRIVE UNIT TRIM TAB Lars E. Granholm, Waukegan, I11.

Outboard Marine Corporation, Waukegan, 111.

Filed: June 26, 1972 Appl. No.: 266,234

Inventor:

Assignee:

U.S. Cl 115/34 R Int. Cl B63h 25/48 Field of Search 115/17, 18, 34, 35;

References Cited UNITED STATES PATENTS 11/1970 l-loltermann et a1. 115/34 R 1/1930 Oertz 12/1931 Costanievich 115/34 R Primary Examiner-Duane A. Reger Assistant Examiner-Charles E. Frankfort Attorney, Agent, or Firm-Michael, Best & Friedrich ABSTRACT 4 Claims, 4 Drawing Figures t/W ,UH I' HUHIUPL STERN DRIVE UNIT TRIM TAB BACKGROUND OF THE INVENTION The invention relates generally to marine propulsion devices. More particularly, the invention relates to stern drive units and to trim tabs or exhaust snouts for such stern drive units.

It has been known for some time that, in a stern drive unit, rotation of the propeller in one rotative direction, i.e., for instance, in the clockwise direction when seen from in front, has a tendency to cause the stern drive unit to steer the supporting boat to starboard. Rotation of the propeller shaft in the opposite direction causes the boat to turn to port. This characteristic of stern drive units has commonly been referred to as resulting from stern drive steering torque."

In order to overcome such steering torque, trim tabs have been employed for some time on stern drive units. Such trim tabs project into the wake of the propeller and have served as rudders to counteract the steering torque developed by the stern drive unit. However, the amount of such counteraction developed by the trim tabs has varied with the speed of the boat. Thus, if the trim tab more or less perfectly counterbalanced or neutralized steering torque at high speeds, there remained an unbalanced condition at low speeds, and vice versa.

The side of the trim tab subject to impact or thrust from the rearwardly revolving stream of water displaced by the propeller is referred to herein as the pressure side.

Examples of prior exhaust snouts or trim tabs are found in the Shimanckas U.S. Pat. No. 3,183,880 issued May I8, 1965, Holterman U.S. Pat. No. 3,537,419 issued Nov. 3, I970, and in the Canadian Kiekhaefer Pat. No. 687,868 issued June 2, I964, as well as in Outboard Marine Print U.S. Pat. No. 312,253 filed with the United States Patent Office concurrently herewith.

SUMMARY OF THE INVENTION The invention provides a stern drive trim tab adapted to approximately neutralize stern drive steering torque under both high and low speed conditions.

More particularly, the invention provides a stern drive unit trim tab which includes a side having one or more vertically extending steps provided by a rearwardly located pressure side portion which, in relation to the forward portion of the pressure side, is located in laterally offset relation toward the other side of the trim tab.

The invention also provides a trim tab wherein both sides thereof are outwardly convexly formed and in which the side opposite the pressure side, i.e., the lift side has a greater convex curvature.

One of the principal objects of the invention is the provision of a trim tab for a stern drive unit, which trim tab operates to more uniformly counterbalance or neutralize stern drive steering torque at both high and low speeds.

Another of the principal objects of the invention is to provide a stern drive unit including a trim tab having forwardly and rearwardly stepped pressure side portions.

Another of the principal objects of the invention is the provision of a trim tab wherein each of the sides is outwardly convexly formed and wherein the convex curvature of the lift side is greater than the pressure side.

Other objects and advantages of the invention will become known by reference to the following description, claims, and drawings.

DRAWINGS FIG. 1 is a fragmentary side elevational view of a stern drive lower unit incorporating various of the features of the invention.

FIG. 2 is a fore and aft sectional view of a portion of the stern drive lower unit shown in FIG. 1 and particularly illustrating the trim tab.

FIG. 3 is a rear end view of the portion of the stern drive lower unit shown in FIG. 2.

FIG. 4 is a bottom view of the portion of the stern drive lower unit shown in FIG. 2.

Before explaining the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts set forth in the following general description or illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

GENERAL DESCRIPTION Shown in the drawings is a stern drive lower unit 11 which includes a drive shaft housing 13 rotatably supporting a drive shaft 15. The stem drive unit can be either a generally upright lower unit as shown, for instancev in the Shimanckas U.S. Pat. No. 3,183,880 issued May IS, 1965 or can be a so-called slant" stern drive unit.

At its lower end, the drive shaft 15 is connected to a propeller shaft 17 by means which can include a reversing transmission 19 of conventional construction. Carried by the propeller shaft 17 is a propeller 21 which is also of conventional construction. In the illustrated construction, the drive shaft 15 is driven by an engine (not shown) in the counterclockwise direction as viewed from above. When in forward drive, the propeller 21 rotates in the clockwise direction when seen from behind and in the counterclockwise direction when seen from in front.

Carried on the drive shaft housing 13 above and rearwardly of the propeller 21 is a trim tab or exhaust snout 23 which can be conventionally employed to discharge exhaust gases into the propeller wake and to include a water inlet for an engine cooling system. In addition, as already noted, trim tabs have been employed to counterbalance steering torque. In this connection, previous trim tabs have sometimes been angularly adjustably mounted as shown in the Canadian Kiekhaefer Pat. No. 687,868 so as to permit selective location thereof depending upon the direction of rotation of the propeller and so as to provide optimum results at a particular speed.

At any rate, for a given direction of propeller rotation, the trim tab 23 includes a pressure side or face 25 against which the rearwardly moving stream of water is driven and a laterally opposed or lift side or face 27.

In accordance with the invention, the trim tab 23 includes means for more or less uniformly neutralizing or approximately counterbalancing steering torque throughout the operating speed range. While other arrangements could be employed, in the illustrated construction, the trim tab 23 is formed so that the pressure side 25 includes a forward portion 29 and a rearward portion 31 which is laterally offset from the forward portion 29 in the direction toward the lift side 27.

While the illustrated construction shows only one step between the forward and rearward portions 29 and 31 of the trim tab pressure side, the trim tab 23 could be provided, in accordance with the invention, with a plurality of steps. v

While a trim tab in accordance with the invention can be constructed without exhaust gas discharge capacity, the illustrated trim tab 23 is provided with a hollow interior which forms a gas passage 33 adapted to communicate with an exhaust gas passage 35 within the drive shaft housing 13. In addition, the hollow interior of the trim tab 23 terminates with a lower exhaust gas outlet 37 at the bottom or lower margin of the trim tab 23 and with a rearward exhaust gas outlet 39 at the rear or aft margin of the trim tab 23. Alternatively, if it is desired, only one of the lower and rearward exhaust gas outlets 37 and 39 could be employed. As a still further alternative, if desired, the lower and rearward outlets 37 and 39 could be integrated into a single outlet extending continuously along the bottom and rear of the trim tab. Other outlet arrangements can also be employed.

Although the illustrated trim tab 23 is not so constructed, a trim tab could be constructed in accordance with the invention with a water inlet for a cooling system.

As is conventional, the trim tab 23 is mounted to the drive shaft housing 13 immediately below the cavitation plate 41 and rearwardly and above the propeller 21 to afford angular adjustment in the position of the trim tab 23 to accommodate forward propeller operation in either rotative direction and to permit location in the position of maximum over-all effectiveness. Such mounting can be accessible solely from within the hollow interior of the trim tab 23 or can be accessible, at least in part, from the exterior of the trim tab 23.

In operation, it is believed that at low speeds, the rearwardly revolving stream of water flowing from the propeller 21 effectively reacts with both the forward and rearward portions 29 and 31 of the pressure side 25 of the trim tab 23 to produce a maximum counterbalancing effect which approximately neutralizes steering torque. At high speed, the rapidly rearwardly moving and revolving stream of water flowing past the pressure side 25 of the trim tab 23 is believed to effectively react with only the forward portion 29 of the pressure side 25 because of the presence of the step between the forward and rearward portions 29 and 31 and the rapidity of the rearward water movement. As a result, the counterbalancing effect which is produced is less than would have occurred in the absence of the step and approximately neutralizes steering torque at high speed. Consequently, the trim tab 23 acts to approximately neutralize steering torque during both high and low speed operation.

Another of the features of the invention resides in the non-symmetry between the pressure and lift sides 25 and 27, respectively of the trim tab 23. In this regard, the forward portion 29 of the pressure side 25 and the lift side 27 are, referring to a fore and aft plane, nonsymmetrical with respect to each other. More specifically, while both the pressure side 25 and the lift side 27 are outwardly convexly curved, the lift side 27 has a greater amount of curvature, i.e., has a shorter radius than the pressure side 25. However, the curvature provided for each side can be compound, i.e., need not extend at a uniform radius from a single center. Such nonsymmetry serves to assist in the operation of the trim tab.

The employment of a substantially outwardly convex form for the lift side 27 serves to enhance the capacity of the trim tab to counter steering torque as the curvature provided on the lift side 27 causes the development of a lower pressure in the water adjacent to the lift side, whereby to increase the tendency of the trim tab to move the stern drive unit in the rotative direction toward the condition of low pressure in the water, and thus augmenting the effect produced as a result of impact of the propeller wake against the pressure side 25. If desired, in order to tailor the steering torque counterbalancing effect of the lift side 27, one or more steps can be provided in the lift side in more or less the same manner as provided in the pressure side 25. Thus, FIG. 5 of the drawings shows, in dotted outline, the provision of a stepped or offset surface 40 on the lift side 27, which surface 40 is located rearwardly on the lift side and is offset in the direction of the pressure side 25.

Various of the features of the invention are set forth in the following claims.

What is claimed is:

l. A stern drive lower unit comprising a lower portion which is at least partially submerged during normal operation and which includes a rotatably mounted propeller shaft, a propeller carried on said propeller shaft for rotation therewith, and a trim tab which is located rearwardly and above said propeller shaft and which includes two side surfaces, one of said side surfaces being subject to impact by water propelled by said propeller and including a forward portion, a rearward portion offset from said forward portion in the direction toward the other of said side surfaces, and a step connecting said forward and rearward portions.

2. A stern drive unit in accordance with claim 1 wherein said trim tab has a bottom margin and a rearward margin and an exhaust gas outlet in one of said bottom and rearward margins.

3. A stern drive unit in accordance with claim 1 wherein both said side surfaces are convexly outwardly curved and wherein the convex outer curvature of said other of said surfaces is greater than the convex outer curvature of said one of said surfaces.

4. A stern drive unit in accordance with claim 3 wherein said other of said surfaces has a forward portion and a rearward portion offset from said forward portion in the direction toward said one of said side surfaces. 

1. A stern drive lower unit comprising a lower portion which is at least partially submerged during normal operation and which includes a rotatably mounted propeller shaft, a propeller carried on said propeller shaft for rotation therewith, and a trim tab which is located rearwardly and above said propeller shaft and which includes two side surfaces, one of said side surfaces being subject to impact by water propelled by said propeller and including a forward portion, a rearward portion offset from said forward portion in the direction toward the other of said side surfaces, and a step connecting said forward and rearward portions.
 2. A stern drive unit in accordance with claim 1 wherein said trim tab has a bottom margin and a rearward margin and an exhaust gas outlet in one of said bottom and rearward margins.
 3. A stern drive unit in accordance with claim 1 wherein both said side surfaces are convexly outwardly curved and wherein the convex outer curvature of said other of said surfaces is greater than the convex outer curvature of said one of said surfaces.
 4. A stern drive unit in accordance with claim 3 wherein said other of said surfaces has a forward portion and a rearward portion offset from said forward portion in the direction toward said one of said side surfaces. 